Hyperopic defocus and diurnal changes in human choroid and axial length

PURPOSE: To investigate the influence of monocular hyperopic defocus on the normal diurnal rhythms in axial length and choroidal thickness of young adults. METHODS: A series of axial length and choroidal thickness measurements (collected at ∼3 hourly intervals, with the first measurement at ∼9 am and the final measurement at ∼9 pm) were obtained for 15 emmetropic young adults over three consecutive days. The natural diurnal rhythms (day 1, no defocus), diurnal rhythms with monocular hyperopic defocus (day 2, -2.00 DS spectacle lens over the right eye), and the recovery from any defocus induced changes (day 3, no defocus) in diurnal rhythms were examined. RESULTS: Both axial length and choroidal thickness underwent significant diurnal changes on each of the three measurement days (p < 0.0001). The introduction of monocular hyperopic defocus resulted in significant changes in the diurnal variations observed in both parameters (p < 0.05). A significant (p < 0.001) increase in the mean amplitude (peak to trough) of change in axial length (mean increase, 0.016 ± 0.005 mm) and choroidal thickness (mean increase, 0.011 ± 0.003 mm) was observed on day 2 with hyperopic defocus compared to the two "no defocus" days (days 1 and 3). At the second measurement (mean time 12:10 pm) on the day with hyperopic defocus, the eye was significantly longer by 0.012 ± 0.002 mm compared to the other two days (p < 0.05). No significant difference was observed in the average timing of the daily peaks in axial length (mean peak time 12:12 pm) and choroidal thickness (21:02 pm) over the three days. CONCLUSIONS: The introduction of monocular hyperopic defocus resulted in a significant increase in the amplitude of the diurnal change in axial length and choroidal thickness that returned to normal the following day after removal of the blur stimulus.